1. Field of the Invention
The present invention relates to a quantum cascade laser (QCL).
2. Description of the Related Art
QCLs are known as semiconductor optical devices which emit light in a mid-infrared wavelength region. The QCLs are promising light sources usable in fields of environmental gas analysis, medical diagnosis, industrial machining, and the like. A QCL having a buried-heterostructure (BH) is described in Applied physics Letters, vol. 94, 011103, (2009).
This buried-heterostructure-type quantum cascade laser (BH-type QCL) includes a mesa waveguide formed on the substrate, a buried layer formed on a side surface of the mesa waveguide, an upper electrode disposed on the masa waveguide, and a lower electrode disposed on the back surface of a substrate. The mesa waveguide includes a contact layer, an upper conductive layer, a core layer, and a lower conductive layer. The buried layer is made of a semi-insulating semiconductor, for example. The core layer has a multi-quantum well (MQW) structure. A plurality of subband levels are formed in the conduction band of the MQW structure in the QCL. In this QCL, an electron transfer pathway constructed from a plurality of subband levels in the conduction band is formed in the core layer by applying a predetermined voltage to the core layer through the upper electrode and the lower electrode. Lasing occurs by transfer of electrons through this transfer pathway from a subband level in the high potential side to a subband level in the low potential side while radiative transition is repeated.